Highly homogeneous zero-index metamaterials make devices more compact and perform better

A highly homogeneous microwave zero-index metamaterial based on high-permittivity SrTiO3 ceramics is demonstrated to realize the small-aperture high-directivity antenna. Such a novel technique is a remarkable step forward to develop compact devices with better performance.

for both methods to satisfy the homogenization criteria of metamaterials 12 .This limitation greatly hinders the development of ZIM-based devices towards compactness, thinness, and better performance.
In a recently published paper in eLight 13 , a collaborative team led by Prof. Yang Li, Prof. Yue Li, Prof. Jingbo Sun, and Prof. Ji Zhou from Tsinghua University, along with Prof. Ming Bai from Beihang University proposed a new approach for designing and realizing the highly homogeneous microwave zero-index metamaterial (ZIM), in which the lattice constant a is only one-tenth of the freespace wavelength λ 0 .Compared to the traditional photonic crystal-based ZIMs with lattice constant of ~λ0 /3, the proposed ZIM's homogenization level is highly improved.This achievement was made by filling highpermittivity SrTiO 3 ceramics with a microwave relative permittivity ε r of over 290 in BaTiO 3 (ε r ≈ 25) background matrix.Based on the highly homogeneous ZIM, the smallaperture high-directivity antenna was designed and realized.
Figure 1 presents a comparative illustration of the conventional ZIM and the highly homogeneous ZIM.In general, the microwave DCZIMs are made by Al 2 O 3 (ε r = 10) pillars in an air matrix 14 , which results in the value of a/λ 0 ranging from 0.3 to 0.5, and thus significantly restricts the ZIM's homogenization level (Fig. 1a).Such a large a/λ 0 limits the minimum surface area of the DCZIM to 1.5λ 0 × 1.5λ 0 because it needs a surface area larger than 3a×3a to behave as a bulk zeroindex medium.To reduce lattice constant and improve uniformity, the authors have synthesized and characterized high-permittivity SrTiO 3 ceramics.Based on the solid-state method with binder, the fabricated SrTiO 3 ceramic showing a nearly dispersiveless ε r from 292 to 294 and a loss tangent from 9 × 10 -4 to 1.45 × 10 -3 at frequencies ranging from 5 to 8 GHz.By using the highpermittivity SrTiO 3 as an inclusion and BaTiO 3 as the background matrix, a highly homogeneous DCZIM in the microwave regime was realized, as shown in Fig. 1b.Because the high-permittivity feature increasing effectively the degenerate wavelengths of the electric monopole mode and the magnetic dipole mode, the DCZIM shows a homogenization level as high as a/λ 0 ≈ 0.1.Both the simulated and measured near fields indicate that the phase of the wave propagating through the DCZIM exhibits an almost perfect uniform distribution.Leveraging these advantages, the authors developed a highdirectivity antenna by incorporating the highly homogeneous ZIM into a metallic waveguide, and results show that this antenna can achieve a directivity as high as 11.2 dB with an aperture size of 1.2λ 0 × 1.2λ 0 .
The work presents an effective technology to implement the ZIMs with high homogenization and small size, which can improve performance and reduce feature size of the metamaterial-enabled devices, such as the aperture of antennas, the compactness of waveguides with arbitrary shapes, and the thickness of cloaks.In addition, the implementation and composite method of the high-permittivity materials could provide a reference for miniaturization of other advanced metamaterial technologies, such as programmable metasurfaces with real-time wave controls [15][16][17] , which can lead to various applications in wireless communications, imaging, and sensing.

Fig. 1
Fig. 1 Schematic of zero-index materials.a Conventional DCZIM needs a surface area larger than 3a×3a to behave as a bulk zero-index medium.b Highly homogeneous DCZIM whose inclusion and background matrix permittivities are 294 and 25, respectively